[PM-6852 ] Fix F-Droid build constant

[fedemkr]

Type of change

• Bug fix
• New feature development
• Tech debt (refactoring, code cleanup, dependency upgrades, etc)
• Build/deploy pipeline (DevOps)
• Other

Objective

Fix CI/CD to for F-Droid build to pass the constant as it's supposed to so AppCenter and Firebase are removed from the final apk.

There seems to be an issue when building for Release configuration where the :/p CustomConstants=FDROID is not added to the DefineConstants or replaced at some point, causing the corresponding packages not to be removed from the build. So the approach was changed by not passing the parameter in the command and enabling the constant in the Directory.Build.props

Code changes

• Directory.Build.props: Added a comment for the FDROID custom constant.
• build.yml: Enabled FDROID custom constant on the Directory.Build.props by replacing the commented line with the uncommented version of it.

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